The invention relates to a hand device for varying the gas pressure in valve-less balls such as more particularly but not exclusively tennis balls.
A hand device for that purpose, as is to be found for example in U.S. Pat. No. 4,114,350, comprises a hollow needle which is designed to penetrate the wall of a ball whose pressure is to be varied, so as to extend into the interior thereof, the hollow needle being in the form of a substantial tubular member which is screwed by means of its base portion into a transverse wall of a substantially cylindrical outside housing. A further cylindrical housing can be introduced into the outside housing, from above, slidably axially relative to the hollow needle, with the bottom of the further cylindrical housing being formed by a base plate which is designed to support the ball when it is introduced laterally into the further housing. Disposed above the ball in the further cylindrical housing is an axially displaceable plate-shaped measuring member which is urged towards the ball by means of a compression spring supported against an upper cover portion of the further housing, in such a way that from the beginning of the operation of varying the pressure of the ball, the ball is in a position of being clearly compressed from above and from below. Below the transverse wall of the substantially cylindrical outside housing, in the handle portion of the device, the outside housing forms a chamber for an aerosol can which is connected to the hollow needle by way of a valve and a supply conduit, through the transverse wall of the housing. When the further housing which accommodates the ball is introduced into the outside housing from above, the hollow needle passes through an axial bore in the above-mentioned base plate and penetrates the wall of the ball which bears against the base plate, to extend into the interior thereof. When thereupon the valve of the aerosol can is operated, a mixture of compressed air and sealing agent which is contained in the aerosol can then flows under pressure through the supply conduit and the hollow needle into the interior of the ball.
Now, while the above-discussed device, when used in relation to tennis balls or balls of a similar size thereto, can still be referred to as a hand device, nonetheless, with its comparatively large outside housing which contains an aerosol can in the handle portion thereof and the further housing which serves to accommodate the ball, that device is very cumbersome to handle and is expensive to produce, by virtue of its complicated structure. The fact that an aerosol mix of air and sealing agent is introduced into the ball through the hollow needle means that a very large amount of sealing agent is required, in order to cover in the interior of the ball, around the hollow needle, the whole of the area of the ball which has been pressed into a flat condition, with a layer of sealing agent of adequate thickness to provide a satisfactory seal so that, when the hollow needle is withdrawn from the ball, a sufficient amount of sealing agent can flow into the comparatively large perforation formed in the ball by the needle, and thereby seal it off again. If the internal pressure of the ball is to be increased only slightly or even reduced, then, after the hollow needle has been pushed into the ball, the ball must first be reduced in pressure to such a degree that thereupon a suitably large amount of the mixture of compressed air and sealing agent can be introduced into the ball from the aerosol can. As the ball does not have any lateral guide in the further housing of the device and as the above-mentioned measuring member which is subjected to the force of a compression spring must remain movable, with a certain amount of lateral play, it is not out of the question that the ball may move away laterally as it is pumped up, so that transverse forces are applied to the hollow needle; such forces can only be carried by a hollow needle which is of suitable thickness. That results in a comparatively large perforation in the wall of the ball, and that means that there is only a low level of reliability in securely re-closing the perforation in the wall of the ball, in the long term, even when using large amounts of sealing agent. Moreover, the content of an aerosol can for use in the above-discussed hand device is only sufficient for a limited number of operations of increasing the pressure in the ball so that that device involves a considerable level of running costs, when it is used frequently. Finally, the propellent gas which is required for aerosol cans to operate generally has a disadvantageous effect in varying the playing properties of the balls when treated in the above-indicated manner, which is another factor that has a serious adverse effect on the utility of the above-described device, in particular for use in relation to tennis balls.